Method for restoring the microstructure of a textured article and for refurbishing a gas turbine blade or vane

ABSTRACT

During a solution heat treatment grain recrystallization may occur of a textured article, especially at the surface rim. The present invention provides a method for restoring the microstructure of a textured article, such as a gas turbine engine blade, which comprises coating the surface of the article with a high temperature stable surface coating or by a controlled development of an oxide scale and subsequently performing a solution heat treatment, thereby maintaining said thermally stable surface coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This non-provisional application claims priority to theprovisional application Serial No. 60/286,662, filed on Apr. 27, 2001,which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to textured articles and inparticular to the restoration of the microstructure of texturedarticles. The invention is further related to the field of refurbishmentof gas turbine airfoils.

BACKGROUND OF THE INVENTION

[0003] An example of the background technology of the present inventionis U.S. Pat. No. 5,611,670 which shows a gas turbine blade made of anickel based superalloy. Superalloys are nickel- or cobalt-based alloys,typically comprising chromium, titanium, tantalum, aluminum, tungstenand other elements, with excellent high temperature resistance, therebymaintaining high strength properties. Accordingly, superalloys arewidely used in high temperature applications where additionally highmechanical strength is required. A typical application is the casting ofairfoils for gas turbines, jet engines as well as stationary gasturbines, e.g. for industrial applications like power generation.Further improvements in mechanical strength is achieved by casting thesuperalloy as a columnar or as a single crystal. A textured article hasno or very few grain boundaries.

[0004] As additional background, European Patent Application EP 1 038982 A1 describes a process for manufacturing single crystal superalloyarticles. After casting the article is subjected to a heat treatment inorder to further improve the mechanical strength. The heat treatment isa high temperature solution heat treatment which homogenizes themicrostructure of the alloy itself formed by different crystal phases.However, this heat treatment may lead to a grain re-crystallizationoccurrence, initiated by dislocations in the crystal structure. Thisgrain re-crystallization destroys locally the single crystal structurewhich may lead to a dramatic decrease in the mechanical strength of thearticle. Accordingly, grain re-crystallization is a cause for rejectionof single crystal castings if present beyond a preset maximum forre-crystallized grains and can result in low yields of acceptableheat-treated single crystal castings. By heat treating in a carburizingatmosphere, carbon is introduced into the casting and forms carbidestherein that reduce or localize the occurrence of grainre-crystallization.

[0005] European Patent EP 0 525 545 B1 describes the refurbishment ofcorroded superalloy articles. In particular gas turbine airfoils aresubjected to corrosion by hot gases. Typically, a corrosion protectivecoating is provided on the body made from the superalloy. Widely usedcoatings are of the MCrAlY type, where M is iron, cobalt and/or nickel,and Y, for yttrium or another rare earth element or another element suchas lanthanum. This type of coating is usually applied by a plasma sprayprocess. However, despite a corrosion protective coating, the airfoilsare still under corrosion and erosion attack which leads to the need forservicing after a certain period of time. Corrosion results fromcontaminants in the fuel and/or air, and oxidation may also occur athigh temperatures. Depending on the conditions of operation, an oxidelayer of varying thickness may form on the surface of the airfoil. Also,and very significantly, sulfur can penetrate into the base material toform sulfides. Furthermore, internal oxides and nitrides may form withinthe metal near the surface.

[0006] Instead of completely exchanging airfoils, it is often a costsaving option to refurbish the airfoils, i.e. providing a new protectivecoating. This requires complete removal of the old coating, which isrealized by applying mechanical stripping as well as chemical treatment,e.g. with acid. After removal of a substantial part of the old coating,the surface is aluminized. Subsequently, the aluminide layer is removed,thereby also removing oxidized and corroded regions at the surface.

[0007] U.S. Pat. No. 5,413,648 discloses a directionally solidifiedarticle with a plastic deformation damage at the surface, which is proneto recrystallization. This problem is overcome by removing a part of thedeformed surface region.

[0008] European patent No. EP 1 036 850 A1 discloses a single crystalhaving a surface coating for preventing recrystallization fracture byreinforcing the grain boundaries. After a heat treatment the texture ofthe article at the surface shows no single crystal structure anymore,because the surface has grain boundaries, which are reinforced by grainboundary strengthening elements like Zr, Hf, B or C.

[0009] U.S. Pat. No. 6,271,668 shows the need of a coating during onestep of refurbishment of gas turbine components, which is applied on thesurface of the component. The coating is heat treated by which a surfaceregion of the article is aluminized. This heat treatment is performed atlow temperatures in order to avoid detrimental diffusion of atoms fromcorrosion products. The coating is removed together with the corrodedlayers before further heat treatments are performed.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method for restoring themicrostructure of a textured article, e.g. a single crystal or adirectionally solidified article, which comprises creating on thesurface of the article a high temperature stable surface coating andsubsequently performing a solution heat treatment, thereby maintainingsaid thermally stable surface coating.

[0011] As stated previously, grain recrystallization may occur during asolution heat treatment of the article. The present invention underliesthe discovery that grain recrystallization occurs at lower temperaturesat the surface of an article compared to bulk regions. The energy neededfor forming new grains with grain boundaries is lower at the surface. Byapplying a coating on the surface and maintaining this surface coatingduring the solution heat treatment, grain recrystallization issuppressed due to the now provided bulk conditions. Accordingly, aneffective solution heat treatment can be processed, thereby restoringthe microstructure of the textured article without introducing grainrecrystallization. A full description of the effect of suppressing grainrecrystallization by surface coating was given by one of the applicantsin the publication “Recrystallization In Single Crystals Of Nickel BaseSuperalloys”, R. Burgel, P. D. Portella, J. Preuhs, Superalloys 2000,edited by T. M. Pollock, pages 229-238, the teaching of which isincorporated herein by reference, in particular with respect to thealloy compositions disclosed in table I (balance Ni) and heat treatmentparameters in table II. TABLE I Cr Co Mo W Ta Nb Al Ti C Hf CMSX-11B12.5 7 .5 5 5 .1 3.6 4.2 — .04 PWA 1483 12.2 9 1.9 3.8 5 — 3.6 4.1 .07 —SRR 99 8.5 5 — 9.5 2.8 — 5.5 2.2 0.2 — CMSX-6 10 5 3 — 2 — 4.8 4.7 — .08

[0012] TABLE II CMSX-11B solutioning, 1204° C./2 h + 1227° C.2/h + 1249°C./3 h SHT* + 1260° C./6 h; heating with 1 K/min age hdn. 1120° C./5 h +870° C./24 h + 760° C./30 h PWA 1483 solutioning* 1260° C./1 h age hdn.1090° C./4 h SRR 99 solutioning* 1270° C./0, 5 h + 1280° C./1 h + 1290°C./2 h + 1300° C./0, 5 h + 1305° C.0, 5 h; heating with 1 K/min age hdn.1080° C./4 h + 870° C./16 h 3 CMSX-6 solutioning* 1227° C./2 h + 1238°C./2 h + 1271° C./2 h + 1277° C./3 h + 1280° C./2 h; heating with 1K/min age hdn. 1080° C./4 h + 870° C./16 h

[0013] The article is preferably made from a superalloy, which may benickel-based or cobalt-based. The microstructure in such a nickel-basedsuperalloy is formed by a y-phase and a γ′-phase. The temperaturerequired for solution heat treatment is considerably high and is atleast the solution temperature of the γ′-phase. By maintaining such atemperature during the solution heat treatment, an effective restoringof the microstructure is achieved. This high temperature normallyincreases the risk of recrystallization. This risk is substantiallyreduced by applying the surface coating before performing the solutionheat treatment. The temperature of the solution heat treatment ispreferably above 1100° C., more preferred above 1150° C. and even morepreferred above 1200° C.

[0014] Preferably, the article is a gas turbine blade or vane. Therejection rate of textured castings because of grain recrystallizationis in particular high for gas turbine blades because very highmechanical stresses resulting from centrifugal forces require highmechanical strength which is strongly influenced and decreased by grainrecrystallization. Moreover thermal fatigue cracking of blades or vanesmay start from non-textured areas.

[0015] Preferably, the surface coating is an aluminide coating, which ispreferably placed on the surface by a Chemical Vapor Deposition (CVD)process, which is well known in the art, e.g. for aluminizing articlesfor establishing corrosion resistance.

[0016] Some preferred composition of the surface coating with its maincomponents are listed here (weight prozent):

[0017] Ni: 24%, Cr: 17%, Al: 45%,

[0018] Ni: 49%, Cr: 4%, Al: 37%,

[0019] Ni: 49%, Cr: 7%, Al: 35%.

[0020] The Al-content should be at least 2 wt % for an aluminidecoating.

[0021] The surface coating could also be for instance an oxide film,e.g. developed by oxidation of the surface of the article, e.g.:alumina, NiO, chromoxide or mixtures of it. The oxidation of the articlecan be performed in air or in certain atmospheres with a given oxygenpartial pressure in a pre-treatment leading to an oxide coating and thenperforming 3a solution heat treatment. The oxidation of the article canalso be performed in one step during heat-up of the solution heattreatment. First an atmosphere suitable for oxidation is present andafter oxidation has occurred a vacuum is applied in order not to oxidisethe blade material too much.

[0022] Nevertheless the whole solution heat treatment can also beperformed in a controlled oxidizing atmosphere. The oxidation of thesurface is especially useful, if the article shows a plastic deformationof the surface, e.g. by strong grinding or other mechanical surface worksuch as shot peening. Nevertheless a thermally stable oxide layer can beapplied to the surface by plasma spraying without oxidation of thesurface of the article.

[0023] The invention is in particular useful for the refurbishment of agas turbine blade. The invention provides a method for refurbishing agas turbine blade made from a textured superalloy body coated with aprotective coating and comprising the following subsequent steps:Coating the protective coating with a high temperature stable surfacecoating; performing a solution heat treatment of the superalloy, therebymaintaining the thermally stable surface coating; removing jointly thesurface coating and the protective coating and finally providing a newprotective coating on the body for the next engine operation period.

[0024] This refurbishment not only includes the re-coating of aprotective coating system but also includes a solution heat treatmentfor re-establishing the full mechanical strength properties of thetextured structure. Since the gas turbine blade was already subjected toerosion and corrosion attack, plastic deformations in the surface rimare likely to occur. As mentioned above, those plastic deformations area source of grain recrystallization. Accordingly, in the past, therefurbishment did not include full solution heat treatment of the gasturbine blade because grain recrystallization would have occurred. Theheat treatment had to be restricted to a lower temperature heattreatment for bonding the new protective coating. Restoring of themicrostructure was not or not completely possible with such a treatment.

[0025] Now, a solution heat treatment can be performed withoutrecrystallization because the surface coating or the oxide film alsocovers areas where the old protective coating is eroded and establishesbulk conditions for the gas turbine blade body which leads to a highertemperature threshold for grain recrystallization.

[0026] Depending on the actual condition of the old protective coating,the following method can be applied instead of the above-mentionedmethod. The following subsequent steps are performed, achieving the sameadvantages mentioned above: removing the protective coating; coating thesurface of the gas turbine blade or vane with a high temperature stablesurface coating; subsequently performing a solution heat treatment,thereby maintaining the thermally stable surface coating; removing thesurface coating and providing a new protective coating on the body.

[0027] The invention is explained in greater detail below and byreference to exemplary embodiments shown in the drawings wherein likenumerals refer to equivalent elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIGS. 1, 2 and 3 show schematically the microstructure of asuperalloy.

[0029]FIG. 4 is a gas turbine blade.

[0030] FIGS. 5 to 11 shows several steps of the method of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention involves heat treating single crystal ordirectionally solidified articles made from a nickel-based superalloy.

[0032] A schematic view of a single crystal article 1 made from anickel-based alloy is shown in FIG. 1. The article has a body 3.According to the single crystal structure, no grain boundaries arepresent. In the case of directionally solidified articles (FIG. 2) thereare grain boundaries 35 only along the z-axis 33. The z-axis is e.g. aradial direction of a turbine blade when in use in a gas turbine. Thegrain boundaries extend mainly along dotted lines along the z-axis 33.

[0033] However, the alloy has a texture formed by mainly two crystalphases 5,7. The first phase 5 is a y-phase, the second phase is aγ′-phase, both crystal structures are well known to those skilled in theart.

[0034] The body 3 has a surface 10 on which a surface coating 11 can bedeposited e.g. by a Chemical Vapor Deposition (CVD) process with a CVDcoating apparatus. The surface coating 11 can be an aluminide coating.Further shown is a lattice defect 9, i.e. a dislocation. Such latticedefects may be caused from mechanical impact on the surface. Themechanical strength of the alloy is influenced by the homogeneity of themicrostructure. Therefore, a solution heat treatment is applied at atemperature or a range of temperatures to the article which meansholding a high temperature for a certain time, e.g. 1 to 5 hours. For anefficient solution heat treatment, the temperature has to be at leastthe solution temperature of the γ′-phase 7. By applying this solutionheat treatment, a restoration of the microstructure is established.

[0035] However, prior art solution heat treatment had as a consequence asignificant risk of introducing grain recrystallization. This means thatareas, i.e. grains, with different lattice orientations develop.Accordingly, grain boundaries are formed which lower the mechanicalstrength. When a certain depth or amount of recrystallization isexceeded, the article has to be rejected.

[0036]FIG. 3 illustrates this recrystallization. A new grain 15 isdeveloped by growth into the original grain 13. A high dislocationconcentration 9 as shown in FIG. 1 is the source for therecrystallization.

[0037] As a discovery underlying the invention, the occurrence ofrecrystallization can be suppressed by the surface coating 11. Theenergy for developing new grains and thereby storing energy in the grainboundaries is higher in the bulk as compared to the surface. The coating11 implements bulk conditions at the surface 10, thereby increasing thethreshold temperature for recrystallization. Accordingly, a solutionheat treatment without recrystallization can be performed.

[0038] The surface coating 11 can also be generated by oxidation duringa heat treatment in air or in a atmosphere with an oxygen partialpressure. The surface coating 11 is generated from the material of asurface rim of the article 1.

[0039] A gas turbine blade 21 made from a single crystal nickel-basedsuperalloy is shown in FIG. 4. The body 23 of the blade 21 is coatedwith a corrosion protective coating 29. On the protective coating 29, athermal barrier coating 31 is placed. After a certain time period ofoperation, the blade 23 needs to be refurbished because the coatingsystem is at least partially worn off by oxidation, corrosion anderosion.

[0040] The thermal barrier coating 31 and/or the protective coating 29is removed by chemical and/or mechanical stripping, e.g. by grinding.During this procedure of mechanical stripping often a surface damagedsurface rim is generated, usually with a depth of 20 μm from thesurface.

[0041] The surface coating 11 is deposited on the remainder of theprotective coating 29, thereby closing the complete surface of the body23. Subsequently the solution heat treatment is applied, as describedabove. After the heat treatment, the surface coating 11, the protectivecoating 29 and the thermal barrier coating 31 are removed by mechanicalstripping and chemical treatment and a new protective coating as well asa new thermal barrier coating are provided.

[0042] Another embodiment, the protective coating 29 and the thermalbarrier coating 31 could be removed first, then the surface coatingdeposited and the solution heat treatment performed. Subsequently, a newprotective coating as well as a new thermal barrier coating areprovided.

[0043] FIGS. 5 to 11 show schematically the steps of restoring themicrostructure of a textured article and/or the refurbishing of atextured article. FIG. 5 shows the article 1 with the body 3 having adislocation accumulation 9. On the body 3 there is e.g. a layer 38 withcorrosion products on which the protective layer 29 and thermal barriercoating 30 is laying.

[0044] In a first step of the method of the invention, the protectivelayer 29, the thermal barrier coating 30 and the corrosion products inthe layer 38 are removed (FIG. 6). Secondly, on the surface 10 of thebody 3 the surface coating 11 is applied by a coating apparatus 42.(FIG.7) Different kinds of coating apparatus or processes can be used. Thesurface coating 11 can also be applied by oxidising the body 3 at highertemperatures HT (FIG. 8) in an atmosphere with oxygen 02. The articlewith the surface coating 11 according to FIG. 7, 8 is now applied to asolution heat treatment as indicated in FIG. 9. The thermal stablesurface coating 11 is maintained during the hole solution heattreatment.

[0045]FIG. 10 shows the article 1 with his body 3 after the solutionhead treatment and removal of the surface coating 11. No grain boundaryinside the single crystal or no additional grain boundary inside thedirectionally solidified article 1, especially in the surface rim ispresent. A new protective layer 29 and a new thermal barrier coating 30can now be applied again on the body 3 of the article 1 (FIG. 11).

[0046] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the invention, and all suchmodifications would be obvious to one skilled in the art intended to beincluded within the scope of the following claims.

What is claimed is:
 1. A method for recovering texture of a texturedarticle comprising the steps of: creating on a surface of the article ahigh temperature stable surface coating; and performing a solution heattreatment on said article, thereby maintaining said thermally stablesurface coating and keeping a textured microstructure.
 2. The methodaccording to claim 1, wherein said article is made from a superalloy. 3.The method according to claim 2,wherein said superalloy is nickel-based.4. The method according to claim 2,wherein said superalloy iscobalt-based.
 5. The method according to claim 3,wherein a γ-phase and aγ′-phase are present in said superalloy and wherein the temperature ofsaid solution heat treatment is at least the solution temperature of theγ′-phase.
 6. The method according to claim 2, wherein said solution heattreatment is performed with a temperature above 1100° C.
 7. The methodaccording to claim 2, wherein the textured article is a single crystalarticle.
 8. The method according to claim 2, wherein the texturedarticle is a directionally solidified article.
 9. The method accordingto claim 1, wherein said article is a gas turbine blade.
 10. The methodaccording to claim 1, wherein said surface coating is an aluminidecoating.
 11. The method according to claim 1, wherein said surfacecoating is an oxide film or scale generated by oxidation of the surface.12. The method according to claim 10, wherein said aluminide coating isprovided by a chemical vapor deposition process.
 13. A method forrefurbishing a gas turbine blade made from a textured superalloy bodycoated with a protective coating, the method comprising the steps of:coating a surface of said body with a high temperature stable surfacecoating, thereby covering said protective coating; performing a solutionheat treatment on the body, thereby maintaining said thermally stablesurface coating; removing jointly said surface coating and saidprotective coating; and providing a second protective coating on saidbody.
 14. The method according to claim 13, wherein a γ-phase and aγ′-phase are present in said superalloy and wherein the temperature ofsaid solution heat treatment is at least the solution temperature of theγ′ phase.
 15. The method according to claim 13, wherein said solutionheat treatment is performed with a temperature above 1100° C.
 16. Amethod for refurbishing a gas turbine blade made from a texturedsuperalloy body coated with a protective coating, the method comprisingthe steps of: removing the protective coating; coating a surface of saidbody with a high temperature stable surface coating; performing asolution heat treatment on said body, thereby maintaining said thermallystable surface coating; removing the surface coating; and providing asecond protective coating on said body.
 17. The method according toclaim 16, wherein a γ-phase and a γ′-phase are present in the superalloyand wherein the temperature of said solution heat treatment is at leasta solution temperature of the γ′-phase.
 18. The method according toclaim 16,wherein said solution heat treatment is performed with atemperature above 1100° C.
 19. The method according to claim 16, whereinthe textured article is a single crystal article.
 20. The methodaccording to claim 16, wherein the textured article is a directionallysolidified article.